An antibody is a principal player in humoral immunity and takes an important role in biological defense as well as a sensitized lymphocyte. Meanwhile, an antibody is frequently used for various kinds of techniques that take advantage of a specific affinity with its antigen, such as affinity chromatography or immunoassays, being indispensable tools in the biotechnology field. Consequently, an antibody is prepared by administering an antigen protein to an animal as an immunogen to induce a humoral immune response. At this time, the antigen protein isolated and purified from a sample such as a biological sample is used as an immunogen, for example. Recently, it is commonly practiced to introduce a gene encoding an antigen protein into a host cell by means of recombinant DNA techniques so as to isolate and purify a recombinant protein from a cultured broth of the host cell. When it is difficult to prepare an antigen protein by means of recombinant DNA techniques, it is also practiced to chemically synthesize a peptide corresponding to a part of the antigen protein and administer the synthesized peptide to the animal as an immunogen.
On the other hand, there is a technique called gene immunization, in which a gene encoding an antigen protein is expressed in the animal to induce an immune response in stead of inoculating the antigen protein. In carrying out gene immunization, a gene encoding an antigen protein is integrated in an appropriate expression vector, for example, so as to inoculate the expression vector to the animal, whereupon the gene integrated in the expression vector is expressed in the animal to synthesize an antigen protein. As a consequence, the antigen protein synthesized in the animal induces an immune response. According to gene immunization, as long as a gene encoding an antigen protein is isolated, immunization is carried out, dispensing with isolation and purification of the antigen protein. Consequently, it is possible to induce an immune response to the antigen protein even if its purification method is unestablished, its purification is difficult, or it is an unknown antigen protein with only its gene being known. That makes it possible to obtain an antibody against such an antigen protein. Further, gene immunization has the additional advantage of being able to induce an immune response even if the amount of an antigen protein synthesized by expression of a gene in the animal is far small than that required for direct administration of the antigen protein. It has further the additional advantage that there is no need to chemically synthesize a peptide antigen separately as in the conventional ways even if an antigen protein is difficult to be prepared by recombinant DNA techniques and that it is only necessary to introduce the full length of a gene into the animal.
As described above, although gene immunization has advantages not shared by the conventional ways, a humoral immune response may not be induced depending on the kind of the antigen protein, resulting in possibly failing to produce an antibody. In the following examples, a humoral immune response may not be induced as well as in the conventional method for immunizing. Specifically, in the case that an antigen protein has extremely high homology to a protein contained endogenously in an immunized animal, the antigen protein is not recognized as a foreign body even though being synthesized in the animal, resulting in possibly failing to induce a humoral immune response. Further, in the case that an antigen protein is labile in the animal, the antigen protein in the animal becomes less in amount, resulting in possibly failing to induce a humoral immune response. Still further, in the case that an antigen protein mainly induces cellular immunity, humoral immunity is difficult to be induced. Yet further, gene immunization has a specific disadvantage of possibly failing to induce humoral immune response due to less amount of an antigen protein resulting from the case that a transduction efficiency of a gene encoding the antigen protein into the animal is low or that expression level of a gene encoding the antigen protein in the animal is low.
In order to solve the problems and drawbacks described above of gene immunization, various ingenuities have been proposed. There is an example (patent document 1) that, in induction of an immune response to urokinase, administration of a urokinase gene in the form of a fusion gene combined with a transmembrane domain gene, instead of administration of the urokinase gene alone, induces a high immune response to urokinase, so as to obtain an antibody against urokinase. It is thought that the high immune response obtained herein is induced because the urokinase region in a fusion protein that is an expression product of the fusion gene is compulsorily arranged on the cell surface.
In the field of vaccines, there is an example (patent document 2) that a fusion gene (chimeric nucleic acid) composed of a gene encoding a heat-shock protein HSP70 and a gene encoding an antigen protein is used as a DNA vaccine. In this example, a gene encoding HSP70 derived from Mycobacterium tuberculosis is used. However, this technique induces antigen-specific cellular immunity (killer T-cell), but does not induce humoral immunity, or antibody production. Meanwhile, there is a report (patent document 3) that the use of a fusion protein composed of HSP70 derived from Mycobacterium tuberculosis and an antigen protein as an immunogen makes the HSP70 to function as a suitable adjuvant and induces antibody production against the antigen protein. These mean that an immune response by a fusion protein composed of HSP70 and an antigen protein has a different mechanism in antigen-presenting cells from an immune response by a fusion gene composed of HSP70 gene and an antigen protein gene. In this way, in the case of induction of a humoral immune response to an antigen protein, immunization using a gene of the antigen protein may not always induce a desired immune response. It is considered to be even higher possible that a desired immune response to the antigen protein is not induced.
Patent document 1: WO 02/08416
Patent document 2: WO 01/29233
Patent document 3: WO 94/29459